The present invention relates generally to the field of nuclear power generators and in particular to a new and useful device for supporting heat transfer tubes in an array in a steam generator of a nuclear plant.
Pressurized water vapor generators or heat exchangers, associated with nuclear power stations and which transfer the reactor produced heat from the primary coolant to the secondary coolant that drives the plant turbines may be as long as 75 feet and have an outside diameter of about 12 feet. Within one of these heat exchangers, straight tubes through which the primary coolant flows may be no more than ⅝ inch in outside diameter, but have an effective length of as long as 52 feet between the tube-end mountings and the imposing faces of the tube sheets. Typically, there may be a bundle of more than 15,000 tubes in one of these heat exchangers. It is clear that there is a need to provide structural support for these tubes in the span between the tube sheet faces to ensure tube separation, adequate rigidity and the like.
U.S. Pat. No. 6,498,827 to Klarner, assigned to Babcock and Wilcox Canada, Ltd. and the entirety of which is hereby incorporated by reference, describes an hour-glassed, broached tube support plate (HBTSP) for use supporting these primary coolant tubes in tube bundles. The plate in the prior application supports and aligns the tubes, and minimizes deposit buildup, while providing gaps for reducing pressure drops across the plate, among other benefits.
Using the HBTSP of the prior application typically results in a tube-to-support gap between the tubes and the support holes through the HBTSP. The tube-to-support gap is in addition to the planned gap between the tube and support hole opening for relieving pressure drop differences. The tube-to-support gap is caused by tolerances of manufacture and is required for assembly of the tube bundles.
In use, the flow of steam and water through the tubes induces vibrations in the tube. The vibration may not be effectively restrained because of the tube-to-support gap. Continual vibration will result in fretting and other vibration damage and reduce the life expectancy of the tube considerably. The small tube-to-support gap also provides a limit flow area.
An HBTSP is expensive to manufacture because of the number of holes and the preferred cross-sectional shape, with three or four recesses and a corresponding number of contact or protruding members in each support hole. Complex and expensive broaching machines and tools are needed to make the plates.
A less expensive and simpler tube support plate in which the vibrations can be eliminated is very desirable.
It is an object of the present invention to provide a support plate for nuclear steam generator tubes which eliminates vibrations in the tubes during use.
It is a further object of the invention to provide a tube support plate which is more easily and economically manufactured than presently known support plates, while maintaining the benefits of the known support plates.
Accordingly, a support for a tube bundle is provided having upper and lower plates with circular opening through holes for a plurality of tubes, and slotted disc springs mounted between the upper and lower plates in each through hole for supporting and contacting a tube inserted in the through hole. The upper and lower plates are bolted together and may be separated by spacing washers. The slotted disc springs each have a frusto-conical shape, with a center opening through which one of the tubes of the tube bundle can be inserted.
The slotted disc springs provide gaps for reducing pressure drops between sides of the support, while having deformable direct contact with the tube sides, which reduces or eliminates tube vibration. The support with slotted disc springs provides the same benefits as known supports, while also reducing stress on the tubes in certain conditions, requiring less complex tooling and manufacture, and thereby the manufacturing cost. By reducing the vibrations in the tubes during use, the supports also help extend the useful life of the tubes in the bundle, and the springs tend to act as fins and enhance the heat transfer effect.